Microsoft Word master thesis report fina doc

4.5.2. Catalyst regeneration 
The catalyst regeneration can be proceeded in-situ or ex-situ the process. In case of in-situ 
regeneration or oxidation, effluents are a non negligible issue even if they do not occur often. 
Indeed these gaseous effluents contain a lot of pollutants that have never really been qualified 
and quantified and the treatment method currently used seems to not be efficient.
Origin of contaminants 
a) Impurities in raw materials 
Feed to selective hydrogenation units come from steam cracking or other olefins sources, 
consequently many impurities are present, that come from upstream processes. In the same 
way, hydrogen contains impurities. Even if these impurities are in very little quantity, they 
can fix themselves on the catalyst. When the catalyst is regenerated or oxidised, all these 
impurities, which are no longer in trace amount as they concentrated, leave the catalyst and go 
out with the gaseous effluent.
These contaminants are various: 
- Sulphur compounds (H
2
S, COS, disulphides and mercaptans) 
- Methanol and oxygenated compounds 
- Nitrogen compounds (HCN, NH
3
, amines …) 
- Inorganic chlorides and other mineral salts 
- Arsenic 
- Mercury
- Chlorhydric acid 
b) Parasite reactions 
As described before, catalyst and operating conditions in selective hydrogenation aim at 
privilege the hydrogenation of dienes or alkynes into olefins and disadvantage other chemical 
reactions that could occur. However dimerization (chemical union of two identical molecules) 
and then oligomerization can occur. It is assumed that 10% of dienes and alkynes are 
dimerized. In case of C
4
hydrogenation, the product of dimerization is called green-oils. These 
molecules (C
4 
to C
20
) coat the catalyst and deactivate it but can be removed by regeneration. 
Catalyst regeneration principle 
During catalyst regeneration procedure, a combustion (or oxidation) is performed in order to 
burn deposited coke (or green oils). Other impurities can also be stripped by this process.
Operations of catalyst regeneration are a bit different depending from the process. 
Nevertheless, main steps are more or less the same: 
- Heating the catalyst bed by circulating heated nitrogen through the reactor. 
- Catalyst bed stripping by establishing a steam circulation through the reactor (this step is not 
always performed). 
- Catalyst pre-oxidation by slowly adding air to the steam. 
- Catalyst impurities burning-off by raising temperature and injecting air again. 
- Cooling and purge with steam first and then nitrogen. 

---

51
The out coming gas is consequently contaminated with CO
2
and CO and also unburned coke 
and other impurities. Here below is a list of compounds that can be found in the effluent 
regeneration gas: 
Compound Maximum 
content 
Green oils 
10 % of the compound that have to be 
hydrogenated. 
Sulphur (H
2
S, SO
2
, SO
3
) (1) 
Mercury (1) 
Arsenic (AsH
3
) (1) 
Chloride (HCl) 
(1) 
(1) The quantity of contaminants deposited on the catalyst can not be known. The only data is 
the maximum content of contaminants in the raw materials. 
Table B-4-2: Compounds possibly present in catalyst regeneration effluents 
Decoking drum 
The most usual way to treat in-situ regeneration gas in hydrogenation processes are currently 
decoking drum. This decoking drum is basically a drum in which the gas is simply washed 
with water before being released in the atmosphere. This washing appears to not be really 
efficient and pollutants are consequently released both in the atmosphere and in the waste 
water. 
 
The following tables summarize effluents from each step of catalyst regeneration for each 
type of process. 
Heating 
Stripping 
Pre-oxidation 
Burning 
Cooling 1 
Cooling 2 
Gas 
components 
N
2 
Steam 
Steam + air 
Steam + air 
Steam 
N
2 
Flowrate (1) (1) 
(1) 
(1) 
(1) 
(1) 
Notice 
Presence 
of HC 
and H
2
O 
Presence 
of HC 
Presence of 
CO
2
and HC, 
impurities (2) 
Presence of 
CO
2
and HC, 
impurities (2) 
Presence of 
air, impurities 
(2) 
- 
Destination Flare (3) 
Flare (3) 
Atmosphere 
via decoking 
drum (3) 
Atmosphere 
via decoking 
drum (3) 
Atmosphere 
via decoking 
drum (3) 
Flare (3) 
(1) Refer to operating instructions 
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount 
in raw materials and which are trapped on the catalyst. 
(3) These are the usual destination but some units can have a special collecting drum. 
Table B-4-3: Gaseous effluents during catalyst regeneration in C
3
 selective hydrogenation 

---

52
Heating 
Oxidation I 
Oxidation II 
Air purge 
Gas 
components 
N
2 
N
2
+ air
O
2
content: 
0,3 % vol 
N
2
+ air
O
2
content: 2 
% vol 
Air 
Flowrate (1) 
(1) (1) 
(1) 
Notice Presence 
of 
HC 
Presence of 
CO
2
and HC, 
impurities (2) 
Presence of 
CO
2
and HC, 
impurities (2) 
- 
Destination Flare 
(3) 
Decoking 
drum (3) 
Decoking 
drum (3) 
Decoking drum 
(3) 
(1) Refer to operating instructions 
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount 
in raw materials and which are trapped on the catalyst. 
(3) These are the usual destination but some units can have a special collecting drum. 
Table B-4-4: Gaseous effluents during catalyst oxidation in C
4
 selective hydrogenation 
First stage reactor: 
Heating 
Stripping 
Pre-
oxidation 
Burning 
Cooling 1 
Cooling 2 
Gas 
components 
N
2 
Steam 
Steam + 
air 
Steam + air 
Steam N
2
Flowrate (1) (1) (1) (1) (1) (1) 
Notice 
Presence of 
HC + H
2
O 
Presence of 
HC 
Presence 
of HC, 
CO
2
Presence of 
HC, CO
2
, 
impurities (2)
- - 
Destination 
Decoking 
drum or 
flare 
Decoking 
drum 
Decoking 
drum 
Decoking 
drum
Decoking 
drum 
Decoking 
drum or 
flare 
(1) Refer to operating instructions 
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount 
in raw materials and which are trapped on the catalyst. 
Table B-4-5: Gaseous effluents during catalyst regeneration in GHU first reactor 

---

53
Second stage reactor: 
Heating 
Stripping 
Pre-
oxidation 
Burning 
Cooling 1 
Cooling 2 
Gas 
components 
N
2 
Steam 
Steam + 
air 
Steam + air 
Steam N
2
Flowrate (1) (1) (1) (1) (1) (1) 
Notice 
Presence of 
HC + H
2
O 
Presence of 
HC, H
2
S 
Presence 
of HC, 
H
2
S, CO
2 
Presence of 
HC, H
2
S, 
CO
2
, 
impurities (2)
- - 
Destination 
Decoking 
drum or 
flare 
Decoking 
drum 
Decoking 
drum 
Decoking 
drum
Decoking 
drum 
Decoking 
drum or 
flare 
(1) Refer to operating instructions 
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount 
in raw materials and which are trapped on the catalyst. 
Table B-4-6: Gaseous effluents during catalyst regeneration in GHU first reactor